I see that Hoosier now lists GT sizes on their website (but they are not listed on tirerack.com yet). It actually looks like they will have two different possible size combinations for the GT. As you may recall, Goodyear also has track tires listed on their website. Note that all the track tires have wider fronts than the OEM tires, to reduce understeer.
I do not know about availability on any of these.
Original Goodyear street tires:
F 235/45-18, 7.7" tread, 26.3" dia. R 315/40-19, 10.2" tread, 28.9" dia.
http://www.tirerack.com/tires/Spec.jsp?make=Goodyear&model=Eagle+F1+Supercar&partnum=345YR8F1SUPERCAR&partnum=14YR9F1SUPERCAR&vehicleSearch=true&index=0&place=0
Goodyear track tires:
F 255/40-18, 9.6" tread, 26.0" dia. R 315/40-19, 12.2" tread, 28.9" dia.
http://www.racegoodyear.com/pdf/2006_price_book/showroom_dot.pdf
Hoosier track tires A: (This set lowers front of car .5")
F 245/40-18, 9.3" tread, 25.3" dia. R 315/40-19, 11.3" tread, 28.9" dia.
Hoosier track tires B: (This set has 2" smaller diameter than OEM tires, which lowers car 1". I'm guessing that these lower aspect ratio, smaller diameter tires would probably probably work best on cars with stiffened suspension. Your speedometer will also read 8% fast with these tires. You would not want to use these for drag racing, because it would probably require a shift to 4th gear in the 1/4 mile.)
F 245/35-18, 9.1" tread, 24.4" dia. R 325/30-19, 11.9" tread, 26.8" dia.
https://www.hoosiertire.com/specrr.htm
